C Clasp
A C clasp is a piece of metal, plastic or other material in the shape of the letter C. The C catches the supporting pin in order to secure the piece of jewelry in place. These types of clasps most often are found on antique brooches. Because C clasps are found most commonly on pieces of jewelry made before 1900, the presence of a C clasp is often a good indicator that the piece of jewelry is more than 100 years old and therefore a true antique. C clasps, while not common, still are used today but are made primarily out of plastic as opposed to metal.
Safety Clasp
The safety clasp effectively replaced the C clasp in 1910. Also known as a rollover clasp or locking C clasp, it has a small piece of metal that rolls over the pin in order to lock the pin in place. It is sturdier and less prone to coming undone than the older C clasp. You can find safety clasps on many pieces of vintage jewelry, and they also are used today in the creation of modern jewelry.
Trombone Clasp
A trombone clasp has a piece of metal on one side that slides in and out, mimicking the action of a trombone. This sliding piece secures the pin in place. Trombone clasps are much more common in Europe than in the United States. They are often a good indicator of an antique piece of jewelry, as they have been used in jewelry-making since the 1850s.